1. Field of the Invention
The present invention relates to a power supply system having a power source to supply power to load circuits through respective power lines.
More particularly, the present invention relates to a power supply system which is capable of cutting off a power line from a load circuit that has caused a failure due to a short circuit (hereinafter, this type of failure will be referred to as a short-circuit failure) to produce an overcurrent, without causing voltage fluctuations, etc., in the other load circuits.
2. Description of the Related Art
Load circuits such as magnetic disk units are required to be compact, and low in production cost. Also, the load circuits are required to have a high efficiency and a high reliability. To satisfy such requirements, it is necessary for a power supply system to have one or more power source circuits which are simultaneously capable of supplying electric power to a plurality of load circuits.
If a short-circuit failure occurs in one of the load circuits in the power supply system, it may adversely affect the other load circuits. To avoid this problem, each load circuit is provided with a protection circuit such as a fuse or a capacitor.
A typical protection circuit is slow to cut off a power line from a failed load circuit, causing a voltage fluctuation in the power line, and therefore involving a troublesome maintenance work. To solve these problems, it is required to provide a power supply system that quickly and surely deals with a short-circuit failure.
Here, to enable some features and problems regarding a conventional power supply system to be understood more clearly, the configuration of each of several power supply systems in the prior art will be explained with reference to FIGS. 1 through 5B.
FIG. 1 is a circuit diagram showing a power supply system according to the first prior art. This system has a power source circuit 3 connected to load circuits 4-1 to 4-n through respective power lines. If one of the load circuits causes a short-circuit failure, an overcurrent flows to the corresponding power line, which adversely affects the other load circuits.
To avoid this, each of the power lines is provided with a fuse F which melts due to an overcurrent. In addition, a capacitor C having a large capacitance is connected between common sections of the positive (+V) and ground (GND) power lines, to prevent the melting of a fuse in a given power line from causing a voltage fluctuation.
FIG. 2 is a circuit diagram showing a first example of a load circuit according to the first prior art. This load circuit drives a three-phase motor 6.
In FIG. 2, the drive circuit has transistors Q1 to Q6 for driving the motor 6, a motor control circuit 7 for controlling the transistors, a current detection resistor R1 for detecting a motor current flowing through the transistors, and a current detection circuit 8 for detecting the motor current according to a voltage generated by the resistor R1.
The load circuit receives electric power from the power source circuit 3 through the positive (+V) and ground (GND) power lines and a fuse F. The motor control circuit 7 sequentially turns ON and OFF the transistors Q1 to Q6 to drive the three-phase motor 6.
When the three-phase motor 6 is driven, a motor current flows through the resistor R1, which generates a terminal voltage corresponding to the motor current between both terminals thereof.
According to this voltage, the current detection circuit 8 detects the motor current and sends a feedback signal to the motor control circuit 7. According to the feedback signal, the motor control circuit 7 drives the motor 6 at a constant speed.
FIG. 3 is a circuit diagram showing a second example of a load circuit according to the first prior art. This load circuit drives a two-terminal motor 10. The load circuit has transistors Q7 to Q10 for driving the two-terminal motor 10, a motor control circuit 7 for controlling the transistors, current detection resistors R3 and R4 for detecting a motor current flowing through the transistors, and a current detection circuit 8 for detecting the motor current according to a voltage generated between both terminals in each of the resistors R3 and R4.
The load circuit receives power from the power source circuit 3 through the positive (+V) and ground (GND) power lines and a fuse F. The motor control circuit 7 sequentially turns ON and OFF the transistors Q7 to Q10, to drive the two-terminal motor 10.
When the two-terminal motor 10 is driven, a motor current flows through the resistors R3 and R4, which generate terminal voltages corresponding to the motor current.
According to these voltages, the current detection circuit 8 detects the motor current and sends a feedback signal to the motor control circuit 7. According to the feedback signal, the motor control circuit 7 drives the two-terminal motor 10 at a constant speed.
If any part, for example, one of the transistors causes a short-circuit failure while the three-phase motor 6 or the two-terminal motor 10 is being driven, an overcurrent flows to the power lines. Then, the fuse F melts to cut off the power lines from the corresponding load circuit, so as to prevent the overcurrent from flowing into the other load circuits.
When the fuse F melts, it fluctuates the voltage of the power lines. This fluctuation is absorbed by the capacitor C, to prevent an adverse effect on the other load circuits.
FIG. 4 is a circuit diagram showing a power supply system according to the second prior art; FIG. 5A is a circuit diagram showing an example of a load circuit according to the second prior art; and FIG. 5B is a time chart for explaining the operation of a power supply system according to the second prior art.
In FIG. 4, load circuits can be respectively connected to power lines and disconnected from the corresponding power lines under an active state. The power supply system of FIG. 4 has a power source circuit 3 and a plurality of load circuits 4-1 to 4-n connected to the power source circuit 3 through respective power lines.
The power lines of each of the load circuits 4-1 to 4-n are provided with a connector 11, which can be coupled and decoupled under an active state.
The load circuit of FIG. 5A is formed on a printed board Pt. The connector 11 is formed at an end of the printed board Pt. The load circuit receives power through the connector 11.
The printed board Pt has the load circuit 4, a capacitor C2, a fuse F, and a current limit resistor R2. An overcurrent generated in the load circuit 4 is stopped by the fuse F.
The connector 11 has male and female pins P1 to P4. The pins P1 and P2 correspond to a positive (+V) power line, the pin P3 corresponds to a signal line S, and the pin P4 corresponds to a ground (GND) power line.
The male pins have the same length, and the female pins formed on the load circuit side have different lengths.
Namely, on the load circuit side, the pins (male pin portions) P1 and P4 have the same length, and the pins P2 and P3 have shorter lengths. These lengths are shown in order by the relation of P1=P4&gt;P2&gt;P3.
When the connector 11 is coupled, i.e., the male pin portions are respectively engaged with the female pin portions, the pins P1 and P4 are connected at first, and the pins P2 and P3 are successively connected. When the connector 11 is decoupled, the pins are disconnected in opposite order.
When the connector 11 is coupled as mentioned above, the pins P1 and P4 are connected at first, to pass a current from the positive (+V) power line to the pin P1, the current limit resistor R2, the fuse F, the capacitor C2, the pin P4, and the ground (GND) power line. If the capacitance of the capacitor C2 and the resistance of the current limit resistor R2 are represented by c2 and r2, respectively, the capacitor C2 is gradually charged at a time constant of r2.times.c2. This suppresses the generation of a rush (surge) current when the connector 11 is coupled.
As the capacitor C2 is charged, the terminal voltage at both terminals of the capacitor C2 increases. When the voltage reaches a given value, the pin P2 is connected, to short-circuit the current limit resistor R2. As a result, the voltage of the capacitor C2 increases to a predetermined value to supply a sufficient operation voltage to the load circuit 4.
Thereafter, the pin P3 is connected to connect the signal line S to the load circuit 4. As a result, external control signals are supplied to the load circuit 4, to start a normal operation of the load circuit 4.
When the connector 11 is decoupled, the pin P3 is disconnected at first, to stop the supply of signals. Then, the pin P2 is disconnected to let the current limit resistor R2 pass a limited current. Thereafter, the pins P1 and P4 are disconnected to entirely cut off the supply of power.
Referring to FIG. 5B, the pins P1 and P4 are connected at the timing corresponding to time t1, and a charging current flows to the capacitor C2 through the current limit resistor R2, as indicated with a portion indicating a voltage level of the capacitor C2 (1).
The pin P2 is connected at the timing corresponding to time t2, and the voltage of the capacitor C2 reaches the predetermined value, to supply a sufficient operation voltage to the load circuit 4. The pin P3 is connected at the timing corresponding to time t3, and external signals are supplied to the load circuit 4, to start a normal operation of the load circuit 4, as indicated with a portion indicating a voltage level of the signal line (2).
The above-mentioned prior art involve the following problems:
(1) If a short-circuit failure occurs in one of the load circuits, an overcurrent flows to melt the fuse F to cut off the power line from the load circuit in question. The fuse takes time to melt, causing a voltage drop or fluctuation in the other load circuits. PA1 (2) If a short-circuit failure occurs in one of the load circuits, the fuse melts to prevent a voltage fluctuation in the other load circuits. Until the fuse melts, it is necessary to maintain a supply voltage within a specified range. PA1 (3) To realize connection of load circuits with power lines under an actual state in the second prior art, it is necessary to install the current limit resistor R2 for each load circuit. This limits the design criteria of the load circuit. The current limit resistor causes a loss which deteriorates efficiency and generates heat thus deteriorating reliability.
To prevent such a voltage drop or fluctuation, the large capacitor C must be arranged in the power lines. The large capacitor is expensive which increases the cost of the power supply system.
In addition, the characteristics of the capacitor deteriorate as it is being used over a long term, to lower the reliability of the system. The capacitor requires, therefore, troublesome periodic maintenance and replacement.
Accordingly, a large capacitor must be installed in the power lines, or a power source of a large capacity must be employed. This increases costs, deteriorates reliability, and limits design criteria.